Description: Cell-free DNA (cfDNA) is DNA that circulates in the plasma, and in pregnant women, cfDNA is both maternal and fetal in origin. Fetal cfDNA can be detected in maternal plasma as early as 7 weeks' gestation and the proportion of fetal cfDNA increases during the pregnancy. Fetal cfDNA is continuously added to maternal circulation and is cleared within hours. Certain noninvasive prenatal screening tests analyze maternal cfDNA to determine the fetal risk for a variety of genetic disorders. The proportion of cfDNA, calculated as fetal cfDNA/total cfDNA and also known as fetal fraction, is one of the reported measures, and in general, samples with a higher fetal fraction yield more sensitive results than those with lower fetal fraction. [PMID: 24466384]